Golf ball

ABSTRACT

A golf ball  2  includes a spherical core  4 , a mid layer  6  outside the core  4 , and a cover  8  outside the mid layer  6 . The core  4  includes a spherical center  10  and an envelope layer  12  outside the center  10 . At all points P included in a zone that extends over a distance range from equal to or greater than 1 mm to equal to or less than 15 mm from the central point of the core  4 , the following mathematical formula is satisfied. 
       −5&lt; H 2− H 1&lt;5
 
     In the mathematical formula, H 1  indicates the JIS-C hardness at a point P 1  located radially inward of each point P at a distance of 1 mm from the point P, and H 2  indicates the JIS-C hardness at a point P 2  located radially outward of each point P at a distance of 1 mm from the point P.

This application claims priority on Patent Application No. 2010-227077filed in JAPAN on Oct. 7, 2010. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the presentinvention relates to multi-piece golf balls that include a center, anenvelope layer, a mid layer, and a cover.

2. Description of the Related Art

Golf players' foremost requirement for golf balls is flight performance.Golf players place importance on flight performance upon shots with adriver, a long iron, and a middle iron. Flight performance correlateswith the resilience performance of a golf ball. When a golf ball havingexcellent resilience performance is hit, the golf ball flies at a highspeed, thereby achieving a large flight distance.

An appropriate trajectory height is required in order to achieve a largeflight distance. A trajectory height depends on a spin rate and a launchangle. In a golf ball that achieves a high trajectory by a high spinrate, a flight distance is insufficient. In a golf ball that achieves ahigh trajectory by a high launch angle, a large flight distance isobtained. Use of a core having an outer-hard/inner-soft structure canachieve a low spin rate and a high launch angle.

Golf players also place importance on spin performance of golf balls.When a backspin rate is high, the run is short. It is easy for golfplayers to cause a golf ball, to which backspin is easily provided, tostop at a target point. When a sidespin rate is high, the golf balltends to curve. It is easy for golf players to intentionally cause agolf ball, to which sidespin is easily provided, to curve. A golf ballto which spin is easily provided has excellent controllability. Inparticular, advanced golf players place importance on controllabilityupon a shot with a short iron.

In light of achieving various performance characteristics, golf ballseach having a multilayer structure have been proposed. JP10-328326 (U.S.Pat. No. 6,468,169) discloses a golf ball that includes a core, anenvelope layer, an inner cover, and an outer cover. JP2001-17575 (U.S.Pat. No. 6,271,296) discloses a golf ball that includes a core, anenvelope layer, a mid layer, and a cover. JP2002-272880 (U.S. Pat. No.6,913,547) discloses a golf ball that includes a core and a cover. Thecore consists of a center and an outer core layer. The cover consists ofan inner cover layer and an outer cover layer. JP2003-205052 (US2003/0166422) discloses a golf ball that includes a center, a mid layer,and a cover. JP2004-130072 (US2004/0029648) discloses a golf ball thatincludes a core and a cover. The core has a three-layer structure.

When a core that has an outer-hard/inner-soft structure and an excessivehardness distribution is hit with a driver, the energy loss is high inthe core. The energy loss impairs resilience performance. When a corethat has an outer-hard/inner-soft structure and an excessive hardnessdistribution is hit with a short iron, the spin rate is low. The lowspin rate impairs controllability.

An object of the present invention is to provide a golf ball thatprovides a large flight distance when being hit with a driver and thathas excellent controllability when being hit with a short iron.

SUMMARY OF THE INVENTION

A golf ball according to the present invention comprises a core, a midlayer positioned outside the core, and a cover positioned outside themid layer. The core comprises a center and an envelope layer positionedoutside the center. A ratio of a volume of the core to a volume of aphantom sphere of the golf ball is equal to or greater than 76%. Adifference (He−Ho) between a JIS-C hardness He at a surface of the coreand a JIS-C hardness Ho at a central point of the core is equal to orgreater than 15 but equal to or less than 30. A JIS-C hardness Hc of thecover is less than the hardness Ho. At all points P included in a zonethat extends over a distance range from equal to or greater than 1 mm toequal to or less than 15 mm from the central point of the core, thefollowing mathematical formula is satisfied.

−5<H2−H1<5

In the mathematical formula, H1 indicates a JIS-C hardness at a point P1that is located radially inward of each point P at a distance of 1 mmfrom the point P, and H2 indicates a JIS-C hardness at a point P2 thatis located radially outward of each point P at a distance of 1 mm fromthe point P.

In the golf ball according to the present invention, a hardnessdistribution of the core is appropriate. The core has a low energy losswhen being hit with a driver. When the golf ball is hit with a driver, alarge flight distance is obtained. The golf ball has excellentcontrollability when being hit with a short iron.

Preferably, the JIS-C hardness Hc of the cover is equal to or less than65. Preferably, a thickness of the cover is equal to or less than 0.8mm.

Preferably, a JIS-C hardness Hm of the mid layer is equal to or greaterthan 90. Preferably, a thickness of the mid layer is equal to or lessthan 1.5 mm.

The cover can be formed from a resin composition. Preferably, aprincipal component of a base material of the resin composition is athermoplastic polyurethane. Preferably, a shear loss elastic modulus G″of the resin composition, which is measured under conditions of avibration frequency of 10 Hz and a temperature of 0° C., is equal to orless than 1.95×10⁷ Pa, and a ratio (E″/G″) of a tensile loss elasticmodulus E″ of the resin composition, which is measured under the sameconditions, to the shear loss elastic modulus G″ is equal to or greaterthan 1.76. Preferably, a polyol component of the thermoplasticpolyurethane is polytetramethylene ether glycol having a number averagemolecular weight of 1500 or less.

Preferably, a diameter of the center is equal to or greater than 10 mmbut equal to or less than 20 mm. Preferably, a thickness of the envelopelayer is equal to or greater than 8 mm but equal to or less than 18 mm.Preferably, a thickness of the mid layer is equal to or less than 1.2mm. Preferably, a thickness of the cover is equal to or less than 0.5mm.

Preferably, a difference (Ho−Hc) between the hardness Ho and thehardness Hc is equal to or greater than 15 but equal to or less than 40.

The center can be formed by a rubber composition being crosslinked. Theenvelope layer can be formed by another rubber composition beingcrosslinked. Preferably, the rubber composition of the center does notinclude an organic sulfur compound, and the rubber composition of theenvelope layer includes an organic sulfur compound.

Preferably, the golf ball further comprises an adhesive layer betweenthe mid layer and the cover. The adhesive layer is formed from anadhesive. A base polymer of the adhesive is a two-component curing typeepoxy resin obtained by curing a bisphenol A type epoxy resin with acuring agent including a polyamine compound. A gel fraction of theadhesive is equal to or greater than 40% but equal to or less than 80%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ballaccording to one embodiment of the present invention;

FIG. 2 is a graph showing a hardness distribution of a core of a golfball according to Example 1 of the present invention;

FIG. 3 is a graph showing a hardness distribution of a core of a golfball according to Example 2 of the present invention;

FIG. 4 is a graph showing a hardness distribution of each core of golfballs according to Examples 3 to 7 of the present invention andComparative Example 4;

FIG. 5 is a graph showing a hardness distribution of a core of a golfball according to Example 8 of the present invention;

FIG. 6 is a graph showing a hardness distribution of a core of a golfball according to Comparative Example 1;

FIG. 7 is a graph showing a hardness distribution of a core of a golfball according to Comparative Example 2; and

FIG. 8 is a graph showing a hardness distribution of a core of a golfball according to Comparative Example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based onpreferred embodiments with reference to the accompanying drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6positioned outside the core 4, and a cover 8 positioned outside the midlayer 6. The core 4 includes a spherical center 10 and an envelope layer12 positioned outside the center 10. On the surface of the cover 8, alarge number of dimples 14 are formed. Of the surface of the golf ball2, a part other than the dimples 14 is a land 16. The golf ball 2includes a paint layer and a mark layer on the external side of thecover 8 although these layers are not shown in the drawing.

The golf ball 2 has a diameter of 40 mm or greater but 45 mm or less.From the standpoint of conformity to the rules established by the UnitedStates Golf Association (USGA), the diameter is preferably equal to orgreater than 42.67 mm. In light of suppression of air resistance, thediameter is preferably equal to or less than 44 mm and more preferablyequal to or less than 42.80 mm. The golf ball 2 has a weight of 40 g orgreater but 50 g or less. In light of attainment of great inertia, theweight is preferably equal to or greater than 44 g and more preferablyequal to or greater than 45.00 g. From the standpoint of conformity tothe rules established by the USGA, the weight is preferably equal to orless than 45.93 g.

Preferably, the center 10 is obtained by crosslinking a rubbercomposition. Examples of preferable base rubbers for use in the rubbercomposition include polybutadienes, polyisoprenes, styrene-butadienecopolymers, ethylene-propylene-diene copolymers, and natural rubbers. Inlight of resilience performance, polybutadienes are preferred. When apolybutadiene and another rubber are used in combination, it ispreferred if the polybutadiene is included as a principal component.Specifically, the proportion of the polybutadiene to the entire baserubber is preferably equal to or greater than 50% by weight and morepreferably equal to or greater than 80% by weight. The proportion ofcis-1,4 bonds in the polybutadiene is preferably equal to or greaterthan 40% and more preferably equal to or greater than 80%.

The rubber composition of the center 10 includes a co-crosslinkingagent. The co-crosslinking agent achieves high resilience of the center10. Examples of preferable co-crosslinking agents in light of resilienceperformance include monovalent or bivalent metal salts of anα,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specificexamples of preferable co-crosslinking agents include zinc acrylate,magnesium acrylate, zinc methacrylate, and magnesium methacrylate. Inlight of releasability of the center from a mold and resilienceperformance of the golf ball, magnesium acrylate and magnesiummethacrylate are preferred.

In light of achievement of both desired releasability of the center froma mold and desired resilience performance of the golf ball 2, the amountof the co-crosslinking agent is preferably equal to or greater than 20parts by weight but equal to or less than 50 parts by weight, per 100parts by weight of the base rubber. The amount is particularlypreferably equal to or greater than 25 parts by weight per 100 parts byweight of the base rubber. The amount is more preferably equal to orless than 45 parts by weight and particularly preferably equal to orless than 40 parts by weight, per 100 parts by weight of the baserubber.

An α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms and ametal oxide may be included in the rubber composition. They both reactwith each other in the rubber composition to obtain a salt. The saltserves as a co-crosslinking agent. Examples of preferableα,β-unsaturated carboxylic acids include acrylic acid and methacrylicacid. Examples of preferable metal oxides include zinc oxide andmagnesium oxide. In light of releasability, magnesium oxide isparticularly preferred. The amount of the α,β-unsaturated carboxylicacid is preferably equal to or greater than 15 parts by weight but equalto or less than 45 parts by weight, per 100 parts by weight of the baserubber. The amount of the α,β-unsaturated carboxylic acid isparticularly preferably equal to or greater than 20 parts by weight per100 parts by weight of the base rubber. The amount of theα,β-unsaturated carboxylic acid is particularly preferably equal to orless than 40 parts by weight per 100 parts by weight of the base rubber.The amount of the metal oxide is preferably equal to or greater than 20parts by weight but equal to or less than 50 parts by weight, per 100parts by weight of the base rubber. The amount of the metal oxide isparticularly preferably equal to or greater than 25 parts by weight per100 parts by weight of the base rubber. The amount of the metal oxide isparticularly preferably equal to or less than 45 parts by weight per 100parts by weight of the base rubber.

Preferably, the rubber composition of the center 10 includes an organicperoxide together with a co-crosslinking agent. The organic peroxideserves as a crosslinking initiator. The organic peroxide contributes tothe resilience performance of the golf ball 2. Examples of suitableorganic peroxides include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of theorganic peroxide is preferably equal to or greater than 0.1 parts byweight, more preferably equal to or greater than 0.2 parts by weight,and particularly preferably equal to or greater than 0.3 parts byweight, per 100 parts by weight of the base rubber. In light of softfeel at impact, the amount of the organic peroxide is preferably equalto or less than 1.5 parts by weight, more preferably equal to or lessthan 1.0 parts by weight, and particularly preferably equal to or lessthan 0.8 parts by weight, per 100 parts by weight of the base rubber.

The rubber composition of the center 10 may include an organic sulfurcompound. Examples of preferable organic sulfur compounds includemonosubstitutions such as diphenyl disulfide,bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide,bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide,bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide,bis(4-cyanophenyl)disulfide, and the like; disubstitutions such asbis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide,bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide,bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide,bis(2-cyano-5-bromophenyl)disulfide, and the like; trisubstitutions suchas bis(2,4,6-trichlorophenyl)disulfide,bis(2-cyano-4-chloro-6-bromophenyl)disulfide, and the like;tetrasubstitutions such as bis(2,3,5,6-tetrachlorophenyl)disulfide andthe like; and pentasubstitutions such asbis(2,3,4,5,6-pentachlorophenyl)disulfide,bis(2,3,4,5,6-pentabromophenyl)disulfide, and the like. The organicsulfur compound contributes to resilience performance. Particularlypreferable organic sulfur compounds are diphenyl disulfide andbis(pentabromophenyl)disulfide.

In light of resilience performance of the golf ball 2, the amount of theorganic sulfur compound is preferably equal to or greater than 0.1 partsby weight and more preferably equal to or greater than 0.2 parts byweight, per 100 parts by weight of the base rubber. In light of softfeel at impact, the amount of the organic sulfur compound is preferablyequal to or less than 1.5 parts by weight, more preferably equal to orless than 1.0 parts by weight, and particularly preferably equal to orless than 0.8 parts by weight, per 100 parts by weight of the baserubber.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the center 10. Examples of suitable fillers include zincoxide, barium sulfate, calcium carbonate, and magnesium carbonate. Theamount of the filler is determined as appropriate so that the intendedspecific gravity of the center 10 is accomplished. A particularlypreferable filler is zinc oxide. Zinc oxide serves not only as aspecific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer,a dispersant, sulfur, an vulcanization accelerator, and the like areadded to the rubber composition of the center 10. Crosslinked rubberpowder or synthetic resin powder may also be dispersed in the rubbercomposition.

In light of resilience performance, the center 10 has a central hardnessHo of preferably 40 or greater, more preferably 45 or greater, andparticularly preferably 50 or greater. In light of suppression of spin,the central hardness Ho is preferably equal to or less than 80, morepreferably equal to or less than 75, and particularly preferably equalto or less than 70. The central hardness Ho is measured by pressing aJIS-C type hardness scale against the central point of a cut plane ofthe center 10 that has been cut into two halves. For the measurement, anautomated rubber hardness measurement machine (trade name “P1”,manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scaleis mounted, is used.

The hardness of the center 10 gradually increases from its central pointtoward its surface. The center 10 has a surface hardness greater thanthe central hardness Ho.

The center 10 has a diameter of preferably 10 mm or greater but 20 mm orless. The center 10 having a diameter of 10 mm or greater can achieveexcellent feel at impact. In this respect, the diameter is morepreferably equal to or greater than 12 mm and particularly preferablyequal to or greater than 13 mm. When the center 10 has a diameter of 20mm or less, the envelope layer 12 having a sufficiently large thicknesscan be formed. In this respect, the diameter is more preferably equal toor less than 18 mm and particularly preferably equal to or less than 17mm.

The envelope layer 12 is obtained by crosslinking a rubber composition.Examples of base rubbers for use in the rubber composition includepolybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers, and natural rubbers. In light ofresilience performance, polybutadienes are preferred. When apolybutadiene and another rubber are used in combination, it ispreferred if the polybutadiene is included as a principal component.Specifically, the proportion of the polybutadiene to the entire baserubber is preferably equal to or greater than 50% by weight and morepreferably equal to or greater than 80% by weight. The proportion ofcis-1,4 bonds in the polybutadiene is preferably equal to or greaterthan 40% and more preferably equal to or greater than 80%.

In order to crosslink the envelope layer 12, a co-crosslinking agent ispreferably used. Examples of preferable co-crosslinking agents in lightof resilience performance include monovalent or bivalent metal salts ofan α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specificexamples of preferable co-crosslinking agents include zinc acrylate,magnesium acrylate, zinc methacrylate, and magnesium methacrylate. Inlight of resilience performance, zinc acrylate and zinc methacrylate areparticularly preferred.

In light of resilience performance of the golf ball 2, the amount of theco-crosslinking agent is preferably equal to or greater than 20 parts byweight and particularly preferably equal to or greater than 25 parts byweight, per 100 parts by weight of the base rubber. In light of softfeel at impact, the amount of the co-crosslinking agent is preferablyequal to or less than 60 parts by weight, more preferably equal to orless than 50 parts by weight, and particularly preferably equal to orless than 45 parts by weight, per 100 parts by weight of the baserubber.

Preferably, the rubber composition of the envelope layer 12 includes anorganic peroxide together with a co-crosslinking agent. The organicperoxide serves as a crosslinking initiator. The organic peroxidecontributes to the resilience performance of the golf ball 2. Examplesof suitable organic peroxides include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of theorganic peroxide is preferably equal to or greater than 0.1 parts byweight, more preferably equal to or greater than 0.3 parts by weight,and particularly preferably equal to or greater than 0.5 parts byweight, per 100 parts by weight of the base rubber. In light of softfeel at impact, the amount of the organic peroxide is preferably equalto or less than 2.0 parts by weight, more preferably equal to or lessthan 1.5 parts by weight, and particularly preferably equal to or lessthan 1.0 parts by weight, per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the envelope layer 12 includes anorganic sulfur compound. The organic sulfur compounds described abovefor the center 10 can be used for the envelope layer 12. In light ofresilience performance of the golf ball 2, the amount of the organicsulfur compound is preferably equal to or greater than 0.1 parts byweight and more preferably equal to or greater than 0.2 parts by weight,per 100 parts by weight of the base rubber. In light of soft feel atimpact, the amount of the organic sulfur compound is preferably equal toor less than 1.5 parts by weight, more preferably equal to or less than1.0 parts by weight, and particularly preferably equal to or less than0.8 parts by weight, per 100 parts by weight of the base rubber.

The golf ball 2 may include a core 4 in which the rubber composition ofthe center 10 does not include an organic sulfur compound and the rubbercomposition of the envelope layer 12 includes an organic sulfurcompound. In the core 4, an appropriate hardness distribution can beobtained.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the envelope layer 12. Examples of suitable fillersinclude zinc oxide, barium sulfate, calcium carbonate, and magnesiumcarbonate. Powder of a metal with a high specific gravity may beincluded as a filler. Specific examples of metals with a high specificgravity include tungsten and molybdenum. The amount of the filler isdetermined as appropriate so that the intended specific gravity of theenvelope layer 12 is accomplished. A particularly preferable filler iszinc oxide. Zinc oxide serves not only as a specific gravity adjusterbut also as a crosslinking activator. According to need, variousadditives such as sulfur, an anti-aging agent, a coloring agent, aplasticizer, a dispersant, and the like are included in the envelopelayer 12 in an adequate amount. Crosslinked rubber powder or syntheticresin powder may also be included in the envelope layer 12.

During formation of the envelope layer 12, the center 10 is covered withtwo uncrosslinked or semi-crosslinked half shells. These half shells arecompressed and heated. By this heating, a crosslinking reaction takesplace to complete the envelope layer 12. The crosslinking temperature isgenerally equal to or higher than 140° C. but equal to or lower than180° C. The time period for crosslinking the envelope layer 12 isgenerally equal to or longer than 10 minutes but equal to or shorterthan 60 minutes.

The hardness of the envelope layer 12 gradually increases from itsinnermost portion to its surface. In light of resilience performance, ahardness He at the surface of the envelope layer 12 (namely, the surfaceof the core 4) is preferably equal to or greater than 70 andparticularly preferably equal to or greater than 75. In light of feel atimpact, the hardness He is preferably equal to or less than 90, morepreferably equal to or less than 88, and particularly preferably equalto or less than 87. The hardness He is measured by pressing a JIS-C typehardness scale against the surface of the core 4. For the measurement,an automated rubber hardness measurement machine (trade name “P1”,manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scaleis mounted, is used.

In light of suppression of spin, the difference (He−Hi) between thesurface hardness He of the envelope layer 12 and a hardness Hi at theinnermost portion of the envelope layer 12 is preferably equal to orgreater than 10, more preferably equal to or greater than 12, andparticularly preferably equal to or greater than 15. In light of ease ofproduction and durability, the difference (He−Hi) is preferably equal toor less than 25.

The hardness Hi is measured for a hemisphere obtained by cutting thecore 4. The hardness Hi is measured by pressing a JIS-C type hardnessscale against the cut plane of the hemisphere. The hardness scale ispressed against a region surrounded by a first circle and a secondcircle. The first circle is the boundary between the center 10 and theenvelope layer 12. The second circle is concentric with the first circleand has a radius larger than the radius of the first circle by 1 mm. Forthe measurement, an automated rubber hardness measurement machine (tradename “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which thishardness scale is mounted, is used.

The envelope layer 12 has a thickness of preferably 8 mm or greater but18 mm or less. The envelope layer 12 having a thickness of 8 mm orgreater can suppress spin. In this respect, the thickness is morepreferably equal to or greater than 9 mm and particularly preferablyequal to or greater than 10 mm. When the envelope layer 12 has athickness of 18 mm or less, the center 10 having a large diameter can beformed. The center 10 having a large diameter can suppress spin. In thisrespect, the thickness is more preferably equal to or less than 16 mmand particularly preferably equal to or less than 15 mm.

In light of suppression of spin, the difference (He−Ho) between thesurface hardness He of the core 4 and the central hardness Ho of thecenter 10 is preferably equal to or greater than 15 and particularlypreferably equal to or greater than 18. In light of ease of productionand resilience performance of the core 4, the difference (He−Ho) ispreferably equal to or less than 30 and particularly preferably equal toor less than 25.

At all points P included in a zone that extends over a distance rangefrom equal to or greater than 1 mm to equal to or less than 15 mm fromthe central point of the core 4, the following mathematical formula issatisfied.

−5<H2−H1<5

In this mathematical formula, H1 indicates the JIS-C hardness at a pointP1. The point P1 is located radially inward of each point P. Thedistance from the point P to the point P1 is 1 mm. In this mathematicalformula, H2 indicates the JIS-C hardness at a point P2. The point P2 islocated radially outward of each point P. The distance from the point Pto the point P2 is 1 mm. The hardnesses H1 and H2 are measured bypressing a JIS-C type hardness scale against a cut plane of the core 4that has been cut into two halves. For the measurement, an automatedrubber hardness measurement machine (trade name “P1”, manufactured byKobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, isused. Preferably, at all the points P included in the zone that extendsover the distance range from equal to or greater than 1 mm to equal toor less than 15 mm from the central point of the core 4, the followingmathematical formula is satisfied.

0<H2−H1<3

The ratio of the volume of the core 4 to the volume of a phantom sphereof the golf ball 2 is equal to or greater than 76%. In other words, thecore 4 is large. The core 4 can achieve excellent resilience performanceof the golf ball 2. The core 4 can suppress spin of the golf ball 2. Inthis respect, the ratio is more preferably equal to or greater than 79%and particularly preferably equal to or greater than 80%. The surface ofthe phantom sphere is the surface of the golf ball 2 when it ispostulated that no dimple 14 exists.

A resin composition is suitably used for the mid layer 6. Examples ofthe base polymer of the resin composition include ionomer resins,styrene block-containing thermoplastic elastomers, thermoplasticpolyester elastomers, thermoplastic polyamide elastomers, andthermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. Ionomer resinsare highly elastic. As described later, the cover 8 of the golf ball 2is thin and flexible. Thus, when the golf ball 2 is hit with a driver,the mid layer 6 significantly deforms. The mid layer 6 including anionomer resin contributes to resilience performance upon a shot with adriver. An ionomer resin and another resin may be used in combination.In this case, in light of resilience performance, the proportion of theionomer resin to the entire base polymer is preferably equal to orgreater than 50% by weight, more preferably equal to or greater than 70%by weight, and particularly preferably equal to or greater than 85% byweight.

Examples of preferable ionomer resins include binary copolymers formedwith an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. A preferable binary copolymer includes 80% by weight ormore but 90% by weight or less of an α-olefin, and 10% by weight or morebut 20% by weight or less of an α,β-unsaturated carboxylic acid. Thebinary copolymer has excellent resilience performance. Examples of otherpreferable ionomer resins include ternary copolymers formed with: anα-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. Apreferable ternary copolymer includes 70% by weight or more but 85% byweight or less of an α-olefin, 5% by weight or more but 30% by weight orless of an α,β-unsaturated carboxylic acid, and 1% by weight or more but25% by weight or less of an α,β-unsaturated carboxylate ester. Theternary copolymer has excellent resilience performance. For the binarycopolymer and the ternary copolymer, preferable α-olefins are ethyleneand propylene, while preferable α,β-unsaturated carboxylic acids areacrylic acid and methacrylic acid. A particularly preferable ionomerresin is a copolymer formed with ethylene and acrylic acid ormethacrylic acid.

In the binary copolymer and the ternary copolymer, some of the carboxylgroups are neutralized with metal ions. Examples of metal ions for usein neutralization include sodium ion, potassium ion, lithium ion, zincion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. Theneutralization may be carried out with two or more types of metal ions.Particularly suitable metal ions in light of resilience performance anddurability of the golf ball 2 are sodium ion, zinc ion, lithium ion, andmagnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”,“Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “HimilanAM7317”, “Himilan AM7318”, “Himilan AM7329”, “Himilan MK7320”, and“Himilan MK7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co.,Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”,“Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “SurlynAD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont deNemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured byExxonMobil Chemical Corporation.

Two or more ionomer resins may be used in combination for the mid layer6. An ionomer resin neutralized with a monovalent metal ion, and anionomer resin neutralized with a bivalent metal ion may be used incombination.

The mid layer 6 may include a highly elastic resin. Examples of highlyelastic resins include polybutylene terephthalate, polyphenylene ether,polyethylene terephthalate, polysulfone, polyethersulfone, polyphenylenesulfide, polyarylate, polyamide imide, polyetherimide, polyether etherketone, polyimide, polytetrafluoroethylene, polyamino bismaleimide,polybisamide triazole, polyphenylene oxide, polyacetal, polycarbonate,acrylonitrile-butadiene-styrene copolymers, and acrylonitrile-styrenecopolymers.

According to need, a coloring agent such as titanium dioxide, a fillersuch as barium sulfate, a dispersant, an antioxidant, an ultravioletabsorber, a light stabilizer, a fluorescent material, a fluorescentbrightener, and the like are included in the mid layer 6 in an adequateamount. For forming the mid layer 6, known methods such as injectionmolding, compression molding, and the like can be used.

The mid layer 6 has a hardness Hm of preferably 90 or greater. The midlayer 6 having a hardness Hm of 90 or greater achieves excellentresilience performance of the golf ball 2. The mid layer 6 having ahardness Hm of 90 or greater can achieve an outer-hard/inner-softstructure of the sphere consisting of the core 4 and the mid layer 6.The sphere having the outer-hard/inner-soft structure suppress spin ofthe golf ball 2. In these respects, the hardness Hm is particularlypreferably equal to or greater than 92. In light of feel at impact, thehardness Hm is equal to or less than 98 and particularly preferablyequal to or less than 97. In light of suppression of spin, preferably,the hardness Hm of the mid layer 6 is greater than the surface hardnessHe of the core 4, and the surface hardness He of the core 4 is greaterthan the surface hardness of the center 10.

The hardness Hm is measured with an automated rubber hardnessmeasurement machine (trade name “P1”, manufactured by Kobunshi KeikiCo., Ltd.) to which a JIS-C type spring hardness scale is mounted. Forthe measurement, a slab formed by hot press and having a thickness ofabout 2 mm is used. A slab kept at 23° C. for two weeks is used for themeasurement. At the measurement, three slabs are stacked. A slab formedfrom a resin composition that is the same as the resin composition ofthe mid layer 6 is used for the measurement.

In light of suppression of spin, the mid layer 6 has a thickness ofpreferably 0.3 mm or greater and particularly preferably 0.5 mm orgreater. In light of feel at impact, the thickness is preferably equalto or less than 1.5 mm, more preferably equal to or less than 1.2 mm,and particularly preferably equal to or less than 1.0 mm.

The cover 8 is formed from a resin composition. Examples of the basepolymer of the resin composition include polyurethanes, polyesters,polyamides, polyolefins, polystyrenes, and ionomer resins. Particularly,polyurethanes are preferred. Polyurethanes are flexible. When the golfball 2 with the cover 8 including a polyurethane is hit with a shortiron, the spin rate is high. The cover 8 formed from a polyurethanecontributes to the controllability upon a shot with a short iron. Thepolyurethane also contributes to the scuff resistance of the cover 8.

When the golf ball 2 is hit with a driver, a long iron, or a middleiron, the sphere consisting of the core 4 and the mid layer 6 becomessignificantly distorted since the head speed is high. Since this spherehas an outer-hard/inner-soft structure, the spin rate is suppressed. Thesuppression of the spin rate achieves a large flight distance. When thegolf ball 2 is hit with a short iron, this sphere becomes less distortedsince the head speed is low. When the golf ball 2 is hit with a shortiron, the behavior of the golf ball 2 mainly depends on the cover 8.Since the cover 8 including the polyurethane is flexible, a high spinrate is obtained. The high spin rate achieves excellent controllability.In the golf ball 2, both desired flight performance upon shots with adriver, a long iron, and a middle iron and desired controllability upona shot with a short iron are achieved.

When the golf ball 2 is hit, the cover 8 including the polyurethaneabsorbs the shock. This absorption achieves soft feel at impact.Particularly, when the golf ball 2 is hit with a short iron or a putter,the cover 8 achieves excellent feel at impact.

When being hit, compressive stress is applied to the cover 8 due tomovement of the head of a golf club. Since the face of the golf club hasa loft angle, shear stress is also applied to the cover 8 when beinghit. The head speed of a short iron is low, and the loft angle of ashort iron is high. Thus, when the golf ball 2 is hit with a short iron,the shear stress greatly influences the deformation behavior of thecover 8. The head speed of a driver is high, and the loft angle of adriver is low. Thus, when the golf ball 2 is hit with a driver, thecompressive stress greatly influences the deformation behavior of thecover 8.

The cover 8 has a shear loss elastic modulus G″ of preferably 1.95×10⁷Pa or less. As described above, when being hit with a short iron, thedeformation behavior of the cover 8 is greatly influenced by the shearstress. The spin rate obtained when being hit with a short ironcorrelates with the shear loss elastic modulus G″. When the golf ball 2with the cover 8 having a shear loss elastic modulus G″ of 1.95×10⁷ Paor less is hit with a short iron, the spin rate is high. The cover 8 canachieve excellent controllability. In this respect, the shear losselastic modulus G″ is particularly equal to or less than 1.83×10⁷ Pa. Inlight of ease of forming the cover 8, the shear loss elastic modulus G″is preferably equal to or greater than 1.00×10⁶ Pa and particularlyequal to or greater than 1.10×10⁶ Pa.

The ratio (E″/G″) of a tensile loss elastic modulus E″ of the cover 8 tothe shear loss elastic modulus G″ is preferably equal to or greater than1.76. As described above, when being hit with a driver, the deformationbehavior of the cover 8 is greatly influenced by the compressive stress.The spin rate obtained when being hit with a driver correlates with thetensile loss elastic modulus E″. When the golf ball 2 with the cover 8having a ratio (E″/G″) of 1.76 or greater is hit with a driver, the spinrate is low, and when the golf ball 2 is hit with a short iron, the spinrate is high. In this respect, the ratio (E″/G″) is more preferablyequal to or greater than 1.86 and particularly preferably equal to orgreater than 1.90. In light of ease of forming the cover 8, the ratio(E″/G″) is preferably equal to or less than 6.0 and particularlypreferably equal to or less than 5.5.

The tensile loss elastic modulus E″ is preferably equal to or greaterthan 2.00×10⁷ Pa, more preferably equal to or greater than 2.20×10⁷ Pa,and particularly preferably equal to or greater than 2.40×10⁷ Pa. Thetensile loss elastic modulus E″ is preferably equal to or less than1.00×10⁸ Pa.

The shear loss elastic modulus G″ and the tensile loss elastic modulusE″ can be controlled by adjusting the molecular weight of a polyol, themolecular weight of a polyisocyanate, a ratio (NCO/OH), and the like.

For measuring the shear loss elastic modulus G″, a sheet having athickness of 2 mm is obtained by press molding from a resin compositionthat is the same as the resin composition of the cover 8. A test piecehaving a width of 10 mm and an inter-clamp distance of 10 mm is punchedout from the sheet. The shear loss elastic modulus G″ is measured forthe test piece. The measurement conditions are as follows.

Apparatus: “Rheometer ARES”, manufactured by TA instruments

Measurement mode: twisting (shearing)

Measurement temperature: 0° C.

Vibration frequency: 10 Hz

Measurement distortion: 0.1%

For measuring the tensile loss elastic modulus E″, a sheet having athickness of 2 mm is obtained by press molding from a resin compositionthat is the same as the resin composition of the cover 8. A test piecehaving a width of 4 mm and an inter-clamp distance of 20 mm is punchedout from the sheet. The tensile loss elastic modulus E″ is measured forthe test piece. The measurement conditions are as follows.

Apparatus: the dynamic viscoelasticity measuring apparatus“Rheogel-E4000”, manufactured by UBM

Measurement mode: pulling

Measurement temperature: 0° C.

Vibration frequency: 10 Hz

Measurement distortion: 0.1%

A time for which a golf ball and a club contact each other is severalhundred microseconds. Thus, the frequency of deformation of the golfball 2 when being hit is several thousand Hz. On average, the golf ball2 is hit at substantially normal temperature (25° C.). On the basis of ageneral time conversion rule of polyurethane, a deformation having afrequency of several thousand Hz in the environment having a temperatureof 25° C. corresponds to a deformation having a frequency of 10 Hz inthe environment having a temperature of 0° C. Thus, in the presentinvention, the shear loss elastic modulus G″ and the tensile losselastic modulus E″ are measured under the conditions of a vibrationfrequency of 10 Hz and a temperature of 0° C.

A polyurethane and another resin may be used in combination for thecover 8. In this case, in light of spin performance and feel at impact,the polyurethane is included as the principal component of the basepolymer. The proportion of the polyurethane to the entire base polymeris preferably equal to or greater than 50% by weight, more preferablyequal to or greater than 70% by weight, and particularly preferablyequal to or greater than 85% by weight.

For the cover 8, thermoplastic polyurethanes and thermosettingpolyurethanes can be used. In light of productivity, thermoplasticpolyurethanes are preferred. A thermoplastic polyurethane includes apolyurethane component as a hard segment, and a polyester component or apolyether component as a soft segment.

The polyurethane includes a polyol component. As the polyol, a polymericpolyol is preferred. Specific examples of polymeric polyols includepolyether polyols such as polyoxyethylene glycol (PEG), polyoxypropyleneglycol (PPG), and polytetramethylene ether glycol (PTMG); condensedpolyester polyols such as polyethylene adipate (PEA), polybutyleneadipate (PBA), and polyhexamethylene adipate (PHMA); lactone polyesterpolyols such as poly-ε-caprolactone (PCL); polycarbonate polyols such aspolyhexamethylene carbonate; and acrylic polyols. Two or more polyolsmay be used in combination.

Particularly, polytetramethylene ether glycol is preferred. The spinrate obtained when the golf ball 2 is hit with a short iron has a highcorrelation with the content of polytetramethylene ether glycol.Meanwhile, the spin rate obtained when the golf ball 2 is hit with adriver has a low correlation with the content of polytetramethyleneether glycol. The golf ball 2 including a polyurethane that includespolytetramethylene ether glycol in an appropriate amount has bothexcellent flight performance when being hit with a driver and excellentcontrollability when being hit with a short iron.

In light of controllability, the polyol has a number average molecularweight of preferably 200 or greater, more preferably 400 or greater, andparticularly preferably 650 or greater. In light of suppression of spin,the molecular weight is preferably equal to or less than 1500, morepreferably equal to or less than 1200, and particularly preferably equalto or less than 850.

The number average molecular weight is measured by gel permeationchromatography. The measurement conditions areas follows.

Apparatus: HLC-8120GPC (manufactured by Tosoh Corporation)

Eluant: tetrahydrofuran

Concentration: 0.2% by weight

Temperature: 40° C.

Column: TSK gel Super HM-M (manufactured by Tosoh Corporation)

Sample volume: 5 microliters

Flow rate: 0.5 milliliter/min

Reference material: polystyrene (“PStQuick Kit-H” manufactured by TosohCorporation)

The polymeric polyol component has a hydroxyl value of preferably 94 mgKOH/g or greater and particularly preferably 112 mg KOH/g or greater.The hydroxyl value is preferably equal to or less than 561 mg KOH/g andparticularly preferably equal to or less than 173 mg KOH/g.

Examples of an isocyanate component in the polyurethane include aromaticpolyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, a mixture (TDI) of 2,4-toluene diisocyanate and2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI),1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate(TODI), xylylene diisocyanate (XDI), tetramethylxylene diisocyanate(TMXDI), and paraphenylene diisocyanate (PPDI); alicyclicpolyisocyanates such as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI),hydrogenated xylylene diisocyanate (H₆XDI), and isophorone diisocyanate(IPDI); and aliphatic polyisocyanates such as hexamethylene diisocyanate(HDI). Two or more polyisocyanates may be used in combination. In lightof weather resistance, TMXDI, XDI, HDI, H₆XDI, IPDI, and H₁₂MDI arepreferred, and H₁₂MDI is particularly preferred.

The polyurethane may include a chain extender as its component. Examplesof chain extenders include low-molecular-weight polyols andlow-molecular-weight polyamines.

Examples of low-molecular-weight polyols include diols, triols,tetraols, and hexaols. Specific examples of diols include ethyleneglycol, diethylene glycol, propanediol, dipropylene glycol, butanediol,neopentyl glycol, pentanediol, hexanediol, heptanediol, and octanediol.Specific examples of triols include glycerin, trimethylolpropane, andhexanetriol. Specific examples of tetraols include pentaerythritol andsorbitol. 1,4-butanediol is preferred.

Examples of low-molecular-weight polyamines include aliphaticpolyamines, monocyclic aromatic polyamines, and polycyclic aromaticpolyamines. Specific examples of aliphatic polyamines includeethylenediamine, propylenediamine, butylenediamine, andhexamethylenediamine. Specific examples of monocyclic aromaticpolyamines include phenylenediamine, toluene diamine, dimethyl toluenediamine, dimethylthio toluene diamine, and xylylenediamine.

The chain extender has a number average molecular weight of preferably30 or greater, more preferably 40 or greater, and particularly 45 orgreater. The molecular weight is preferably equal to or less than 400,more preferably equal to or less than 350, and particularly preferablyequal to or less than 200. Low-molecular-weight polyols andlow-molecular-weight polyamines that are used as chain extenders arelow-molecular-weight compounds that almost do not have a molecularweight distribution. Thus, the low-molecular-weight polyols and thelow-molecular-weight polyamines can be distinguished from the polymericpolyol.

The cover 8 may be formed from a composition including a thermoplasticpolyurethane and an isocyanate compound. During or after forming thecover 8, the polyurethane is crosslinked with the isocyanate compound.

According to need, a coloring agent such as titanium dioxide, a fillersuch as barium sulfate, a dispersant, an antioxidant, an ultravioletabsorber, a light stabilizer, a fluorescent material, a fluorescentbrightener, and the like are included in the cover 8 in an adequateamount.

The cover 8 has a JIS-C hardness Hc of 65 or less. Use of the flexiblecover 8 can achieve excellent controllability upon a shot with a shortiron. In light of controllability, the hardness Hc is more preferablyequal to or less than 60, even more preferably equal to or less than 55,and particularly preferably equal to or less than 50. If the hardness Hcis excessively low, the flight performance upon a shot with a driver isinsufficient. In this respect, the hardness Hc is preferably equal to orgreater than 20, more preferably equal to or greater than 25, andparticularly preferably equal to or greater than 35. For measuring thehardness Hc, a slab formed from a resin composition that is the same asthe resin composition of the cover 8 is used. The measurement method isthe same as the measurement method for the hardness Hm of the mid layer6.

The hardness Hc of the cover 8 is less than the central hardness Ho ofthe core 4. The golf ball 2 has excellent controllability upon a shotwith a short iron. In light of controllability, the difference (Ho−Hc)is preferably equal to or greater than 15, more preferably equal to orgreater than 17, and particularly preferably equal to or greater than20. The difference (Ho−Hc) is preferably equal to or less than 40, morepreferably equal to or less than 35, and particularly preferably equalto or less than 30.

In light of flight performance upon a shot with a driver, the cover 8has a thickness of preferably 0.8 mm or less, more preferably 0.6 mm orless, even more preferably 0.5 mm or less, and particularly preferably0.4 mm or less. In light of controllability upon a shot with a shortiron, the thickness is preferably equal to or greater than 0.10 mm andparticularly preferably equal to or greater than 0.15 mm.

For forming the cover 8, known methods such as injection molding,compression molding, and the like can be used. When forming the cover 8,the dimples 14 are formed by pimples formed on the cavity face of amold.

In light of feel at impact, the golf ball 2 has an amount of compressivedeformation of preferably 2.3 mm or greater, more preferably 2.4 mm orgreater, and particularly preferably 2.5 mm or greater. In light ofresilience performance, the amount of compressive deformation ispreferably equal to or less than 3.5 mm, more preferably equal to orless than 3.2 mm, and particularly preferably equal to or less than 3.0mm.

At measurement of the amount of compressive deformation, first, the golfball 2 is placed on a hard plate made of metal. Next, a cylinder made ofmetal gradually descends toward the golf ball 2. The golf ball 2,squeezed between the bottom face of the cylinder and the hard plate,becomes deformed. A migration distance of the cylinder, starting fromthe state in which an initial load of 98 N is applied to the golf ball 2up to the state in which a final load of 1274 N is applied thereto, ismeasured.

If the cover 8 is laminated directly on the mid layer 6, the cover 8does not firmly adhere to the mid layer 6 due to the difference betweenthe material of the cover 8 and the material of the mid layer 6. Thegolf ball 2 preferably includes an adhesive layer between the mid layer6 and the cover 8. The adhesive layer firmly adheres to the mid layer 6and also to the cover 8. The adhesive layer suppresses separation of thecover 8 from the mid layer 6. As described above, the cover 8 of thegolf ball 2 is thin. When the golf ball 2 is hit by the edge of aclubface, a wrinkle is likely to occur. The adhesive layer suppressesoccurrence of a wrinkle. The golf ball 2 is unlikely to break even bybeing repeatedly hit. In the golf ball 2, loss of energy transfer issmall when the golf ball 2 is hit with a golf club. Thus, the golf ball2 has excellent resilience performance.

The adhesive layer is formed by applying an adhesive to the surface ofthe mid layer 6 and drying the adhesive. The base polymer of theadhesive is a two-component curing type epoxy resin. A preferabletwo-component curing type epoxy resin is obtained by curing a bisphenolA type epoxy resin with a curing agent including a polyamine compound.The bisphenol A type epoxy resin is used for the two-component curingtype epoxy resin, and thus the two-component curing type epoxy resin hasexcellent flexibility, chemical resistance, heat resistance, andtoughness.

The adhesive is obtained by mixing a base material including a bisphenolA type epoxy resin and a solvent with a curing agent including apolyamine compound and a solvent. Examples of the solvents in the basematerial and the curing agent include organic solvents such as xyleneand toluene and water.

Specific examples of the polyamine compound include polyamide amines andmodified products thereof. A polyamide amine has a plurality of aminogroups and one or more amide groups. The amino groups can react withepoxy groups. A polyamide amine can be obtained by a condensationreaction of a polymerized fatty acid and a polyamine. A typicalpolymerized fatty acid is obtained by heating and combining naturalfatty acids including a large amount of unsaturated fatty acids, such aslinoleic acid, linolenic acid, and the like, in the presence of acatalyst. Specific examples of unsaturated fatty acids include tall oil,soybean oil, linseed oil, and fish oil. A hydrogenated polymerized fattyacid having a dimer content of 90% by weight or greater and a trimercontent of 10% by weight or less is preferred. Examples of preferablepolyamines include polyethylene diamines, polyoxyalkylene diamines, andderivatives thereof.

The adhesive has a gel fraction of 40% or greater. In the adhesive layerformed from the adhesive having a gel fraction of 40% or greater, avolatile component is unlikely to remain and thus there are few bubblesincluded. The adhesive layer firmly adheres to the mid layer 6 and alsoto the cover 8. In this respect, the gel fraction is more preferablyequal to or greater than 45% and particularly preferably equal to orgreater than 50%.

The adhesive has a gel fraction of 80% or less. The adhesive having agel fraction of 80% or less sufficiently reacts with the base polymer ofthe mid layer 6 and also with the base polymer of the cover 8. Theadhesive layer firmly adheres to the mid layer 6 and also to the cover8. In this respect, the gel fraction is more preferably equal to or lessthan 76% and particularly preferably equal to or less than 70%.

The adhesive layer formed from the adhesive of which the gel fraction isequal to or greater than 40% but equal to or less than 80% exerts aremarkable effect in the golf ball 2 including the thin cover 8. Theadhesive layer formed from the adhesive of which the gel fraction isequal to or greater than 40% but equal to or less than 80% exerts aremarkable effect in the golf ball 2 including the flexible cover 8.

At measurement of the gel fraction, immediately after the base materialand the curing agent are mixed, the adhesive is applied to a PB-137Tzinc phosphate treated steel plate. The size of the steel plate is “150mm×70 mm”. The thickness of the steel plate is 0.8 mm. The steel plateis kept in the environment of 40° C. for 24 hours to form a coating filmformed from the adhesive. A test piece is obtained from the steel plateand the coating film. The weight of the test piece is measured, and aweight M1 of the coating film is calculated by subtracting the weight ofthe steel plate from the measurement value. The test piece is immersedin acetone and allowed to stand for 24 hours. The test piece is kept inthe environment of 105° C. for one hour. The test piece is cooled to 23°C. The weight of the test piece is measured, and a weight M2 of thecoating film is calculated by subtracting the weight of the steel platefrom the measurement value. A gel fraction G is calculated by thefollowing mathematical formula.

G=(M2/M1)·100

The ratio of the epoxy equivalent of the bisphenol A type epoxy resin tothe amine active hydrogen equivalent of the curing agent in the adhesiveis preferably equal to or greater than 2.0/1.0 but equal to or less than13.0/1.0. In the adhesive in which the ratio is equal to or greater than2.0/1.0, the gel fraction is not too low. Therefore, the adhesive layerfirmly adheres to the mid layer 6 and the cover 8. In this respect, theratio is more preferably equal to or greater than 2.6/1.0 andparticularly preferably equal to or greater than 4.0/1.0. In theadhesive in which the ratio is equal to or less than 13.0/1.0, the gelfraction is not too high. Therefore, the adhesive layer firmly adheresto the mid layer 6 and the cover 8. In this respect, the ratio is morepreferably equal to or less than 12.2/1.0 and particularly preferablyequal to or less than 10.0/1.0.

The amine active hydrogen equivalent of the curing agent is preferablyequal to or greater than 100 g/eq but equal to or less than 800 g/eq. Inthe adhesive in which the equivalent is equal to or greater than 100g/eq, the gel fraction is not too high. Therefore, the adhesive layerfirmly adheres to the mid layer 6 and the cover 8. In this respect, theequivalent is more preferably equal to or greater than 200 g/eq andparticularly preferably equal to or greater than 300 g/eq. In theadhesive in which the equivalent is equal to or less than 800 g/eq, thegel fraction is not too low. Therefore, the adhesive layer firmlyadheres to the mid layer 6 and the cover 8. In this respect, theequivalent is more preferably equal to or less than 600 g/eq andparticularly preferably equal to or less than 500 g/eq.

The adhesive includes water as a volatile component. The term “volatilecomponent” means both water and an organic solvent. The proportion Pw ofwater to the entire volatile component is preferably equal to or greaterthan 90% by weight. In the adhesive in which the proportion Pw is equalto or greater than 90% by weight, the gel fraction is easily controlled.In this respect, the proportion Pw is more preferably equal to orgreater than 95% by weight and particularly preferably equal to orgreater than 99% by weight. The proportion Pw may be 100% by weight. Inlight of the environment, the proportion Po of the organic solvent tothe entire volatile component is preferably equal to or less than 10% byweight, more preferably equal to or less than 5% by weight, andparticularly preferably equal to or less than 1% by weight.

The adhesive layer may include additives such as a coloring agent(typically, titanium dioxide), an antioxidant, a light stabilizer, afluorescent brightener, an ultraviolet absorber, an anti-blocking agent,and the like. The additives may be added to the base material or thecuring agent.

As described above, the adhesive layer is obtained by applying theadhesive to the surface of the mid layer 6. The application can beconducted by a spray gun method, an electrostatic coating method, or adipping method. In light of workability, the application by the spraygun method is preferred. After the application, the solvent isvolatilized to permit a reaction of the bisphenol A type epoxy resinwith the polyamine compound, thereby forming the adhesive layer.

In light of durability of the golf ball 2, the adhesive layer has athickness of preferably 0.001 mm or greater and particularly preferably0.002 mm or greater. The thickness is preferably equal to or less than0.1 mm. The thickness is measured by observing a cross-section of thegolf ball 2 with a microscope. When the mid layer 6 has concavities andconvexities on its surface from surface roughening, the thickness ismeasured at a convex part. The measurement is conducted so as to avoidthe positions of the dimples 14.

The adhesive strength between the mid layer 6 (first layer) and thecover 8 (second layer) is preferably equal to or greater than 20 N. Thegolf ball 2 in which the adhesive strength is equal to or greater than20 N has excellent durability. In this respect, the adhesive strength ismore preferably equal to or greater than 22.0 N and particularlypreferably equal to or greater than 22.3 N.

At measurement of the adhesive strength, a test piece including thefirst layer, the adhesive layer, and the second layer is cut out fromthe golf ball 2. The size of the test piece is “10 mm×50 mm”. At an endof the test piece, the second layer is peeled from the first layer. Thefirst layer is fixed to a first chuck, and the second layer is fixed toa second chuck. The second chuck is moved relative to the first chuck topeel the first layer from the second layer. The force applied at thispeeling is measured. For the measurement, “autograph AG-IS” manufacturedby SHIMADZU CORPORATION is used. The tensile rate is 50 mm/min.

Also for a golf ball that includes a center, an envelope layer formedfrom a resin composition and covering the center, a mid layer formedfrom a resin composition and covering the envelope layer, and a coverformed from a resin composition and covering the mid layer, an adhesivelayer is effective. In the golf ball, an adhesive layer may be presentbetween the envelope layer and the mid layer, and an adhesive layer maybe present between the mid layer and the cover. When the adhesive layeris present between the envelope layer and the mid layer, the envelopelayer is a first layer, and the mid layer is a second layer. When theadhesive layer is present between the mid layer and the cover, the midlayer is a first layer, and the cover is a second layer.

EXAMPLES Example 1

A rubber composition (1) was obtained by kneading 100 parts by weight ofa high-cis polybutadiene (trade name “BR-730”, manufactured by JSRCorporation), 23 parts by weight of zinc diacrylate, 5 parts by weightof zinc oxide, an appropriate amount of barium sulfate, 0.5 parts byweight of diphenyl disulfide, and 0.7 parts by weight of dicumylperoxide. The rubber composition (1) was placed into a mold includingupper and lower mold halves each having a hemispherical cavity, andheated at 170° C. for 15 minutes to obtain a center with a diameter of15 mm.

A rubber composition (2) was obtained by kneading 100 parts by weight ofa high-cis polybutadiene (the aforementioned “BR-730”), 33 parts byweight of zinc diacrylate, 5 parts by weight of zinc oxide, anappropriate amount of barium sulfate, 0.3 parts by weight ofbis(pentabromophenyl)disulfide, and 0.9 parts by weight of dicumylperoxide. Half shells were formed from the rubber composition (2). Thecenter was covered with two half shells. The center and the half shellswere placed into a mold including upper and lower mold halves eachhaving a hemispherical cavity, and heated at 150° C. for 20 minutes toobtain a core with a diameter of 39.7 mm. An envelope layer was formedfrom the rubber composition (2). The amount of barium sulfate wasadjusted such that the specific gravity of the envelope layer agreeswith the specific gravity of the center and the weight of a golf ball is45.4 g.

A resin composition (a) was obtained by kneading 50 parts by weight ofan ionomer resin (the aforementioned “Surlyn 8945”) and 50 parts byweight of another ionomer resin (the aforementioned “Himilan AM7329”)with a twin-screw kneading extruder. The core was placed into a moldincluding upper and lower mold halves each having a hemisphericalcavity. The core was covered with the resin composition (a) by injectionmolding to form a mid layer with a thickness of 1.0 mm.

An adhesive including a base material and a curing agent was prepared.The base material is a water-based epoxy composition manufactured bySHINTO PAINT CO., LTD. The base material includes 36 parts by weight ofa bisphenol A type epoxy resin and 64 parts by weight of water. Theepoxy equivalent of the base material is 1405 g/eq. The curing agent isa water-based amine composition manufactured by SHINTO PAINT CO., LTD.The curing agent includes 44 parts by weight of a modified polyamideamine, 50 parts by weight of water, 1 parts by weight of propyleneglycol, and 5 parts by weight of titanium dioxide. The active hydrogenequivalent of the curing agent is 348 g/eq. This adhesive was applied tothe surface of the mid layer with a spray gun, and kept at 23° C. for 12hours to obtain an adhesive layer with a thickness of 0.003 mm.

A resin composition (b) was obtained by kneading 100 parts by weight ofa polyurethane (trade name “Elastollan XNY85A”, manufactured by BASFJapan Ltd.) and 4 parts by weight of titanium dioxide with a twin-screwkneading extruder. Half shells were obtained from the resin composition(b) by compression molding. The sphere consisting of the core, the midlayer, and the reinforcing layer was covered with two of these halfshells. The sphere and the half shells were placed into a final moldthat includes upper and lower mold halves each having a hemisphericalcavity and that has a large number of pimples on its cavity face. Acover was obtained by compression molding. The cover had a thickness of0.5 mm. Dimples having a shape that was the inverted shape of thepimples were formed on the cover. A clear paint including atwo-component curing type polyurethane as a base material was applied tothis cover to obtain a golf ball of Example 1 with a diameter of 42.7mm.

Examples 2 to 7 and Comparative Examples 1 to 4

Golf balls of Examples 2 to 7 and Comparative Examples 1 to 4 wereobtained in the same manner as Example 1, except the specifications ofthe center, the envelope layer, the mid layer, and the cover were asshown in Tables 5 to 7 below. The rubber composition of the core isshown in detail in Table 1 below. The resin compositions of the midlayer and the cover are shown in detail in Table 2 below. A hardnessdistribution of the core is shown in Tables 3 and 4 below. The golf ballaccording to Comparative Example 1 does not have an envelope layer.

[Shot with Driver (W#1)]

A driver with a titanium head (trade name “SRIXON Z-TX”, manufactured bySRI Sports Limited, shaft hardness: X, loft angle: 8.5°) was attached toa swing machine manufactured by Golf Laboratories, Inc. A golf ball washit under the condition of a head speed of 50 m/sec. The ball speed andthe spin rate immediately after the hit and the distance from the launchpoint to the stop point were measured. The average value of dataobtained by 12 measurements is shown in Tables 5 to 7 below.

[Shot with Middle Iron (I#5)]

A 5-iron (trade name “SRIXON Z-TX (steel shaft), manufactured by SRISports Limited) was attached to the swing machine. A golf ball was hitunder the condition of a head speed of 41 m/sec. The ball speed and thespin rate immediately after the hit and the distance from the launchpoint to the stop point were measured. The average value of dataobtained by 12 measurements is shown in Tables 5 to 7 below.

[Shot with Short Iron (SW)]

A sand wedge (trade name “CG15 Chrome Wedge”, manufactured by SRI SportsLimited, loft angle: 58°) was attached to the swing machine. A golf ballwas hit under the condition of a head speed of 21 m/sec, and the spinrate was measured immediately after the hit. The average value of dataobtained by 12 measurements is shown in Tables 5 to 7 below. Inaddition, water was applied to a clubface and a golf ball, and the golfball was hit. The spin rate was measured immediately after the hit. Theaverage value of data obtained by 12 measurements is shown in Tables 5to 7 below.

[Feel at Impact]

Ten golf players hit golf balls with sand wedges, and were asked aboutfeel at impact. The evaluation was categorized as follows on the basisof the number of golf players who answered, “the feel at impact wasexcellent”.

A: 8 or more

B: 6 to 7

C: 4 to 5

D: 3 or less

The results are shown in Tables 5 to 7 below.

TABLE 1 Composition of Core (1) (2) (3) (4) (5) (6) BR730 100 100 100100 100 100 Zinc diacrylate 23 33 — 30 35 30 Methacrylic acid — — 28.0 —— — Magnesium oxide — — 34.8 — — — Zinc oxide 5 5 — 5 5 5 Bariumsulfate * * — * * * Diphenyl disulfide 0.5 — — — — —Bis(pentabromophenyl) — 0.3 — 0.3 0.3 0.3 disulfide Dicumyl peroxide 0.70.9 1.3 1.05 0.9 0.9 * Appropriate amount

TABLE 2 Compositions of Mid Layer and Cover (a) (b) (c) (d) (e) (f) (g)Surlyn 8945 50 — — — — — — Himilan AM7329 50 — — — — — — Polyurethane *1— 100 — — — — — Polyurethane *2 — — 100 — — — — Polyurethane *3 — — —100 — — — Polyurethane *4 — — — — 100 — — Polyurethane *5 — — — — — 100— Polyurethane *6 — — — — — — 100 Titanium dioxide — 4 4 4 4 4 4Molecular weight — 1800 1800 1800 1500 1500 1000 of PTMG E″ (×10⁷ Pa) —3.24 4.61 7.57 1.80 3.51 6.26 G″ (×10⁷ Pa) — 2.77 3.23 4.01 1.02 1.423.00 E″/G″ — 1.17 1.43 1.89 1.76 2.47 2.09 Hardness (JIS-C) 94 47 56 6745 47 44 Hardness (Shore D) 64 32 38 47 30 32 29

The details of the polyurethanes in Table 2 are as follows. Polyurethane*1

Trade name “Elastollan XNY85A”, manufactured by BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1800 Polyurethane*2

Trade name “Elastollan XNY90A”, manufactured by BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1800 Polyurethane*3

Trade name “Elastollan XNY97A”, manufactured by BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1800 Polyurethane*4

Thermoplastic polyurethane elastomer

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1500 Polyurethane*5

Thermoplastic polyurethane elastomer

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1500 Polyurethane*6

Thermoplastic polyurethane elastomer

Polyol component: polytetramethylene ether glycol

Number average molecular weight of polyol component: 1000

TABLE 3 Hardness Distribution of Core (JIS-C) Distance from centralpoint Example (mm) 1 2 3 4 5 6 7 8 0 65.0 65.0 65.0 65.0 65.0 65.0 65.065.0 5.0 67.0 67.0 67.0 67.0 67.0 67.0 67.0 67.0 6.5 68.0 — 68.0 68.068.0 68.0 68.0 68.0 7.5 68.5 — 68.5 68.5 68.5 68.5 68.5 68.5 7.5 70.5 —70.5 70.5 70.5 70.5 70.5 67.5 8.0 — 68.7 — — — — — — 8.5 72.0 — 72.072.0 72.0 72.0 72.0 69.0 9.0 — 69.5 — — — — — — 9.0 — 72.0 — — — — — —10.0 74.0 73.5 74.0 74.0 74.0 74.0 74.0 71.0 15.0 81.5 81.5 81.5 81.581.5 81.5 81.5 78.5 19.9 86.0 — — — — — — 83.0 20.0 — 86.0 86.0 86.086.0 86.0 86.0 —

TABLE 4 Hardness Distribution of Core (JIS-C) Distance from centralpoint Comparative Example (mm) 1 2 3 4 0 66.0 65.0 65.0 65.0 5.0 72.066.5 67.0 67.0 6.5 — — 68.0 68.0 7.5 — — 68.5 68.5 7.5 — — 67.5 70.5 8.5— — 69.0 72.0 10.0 72.0 69.5 71.0 74.0 12.5 — 71.5 — — 12.5 — 75.0 — —15.0 78.0 79.0 78.5 81.5 19.5 — 87.0 83.0 — 20.0 84.0 — — 86.0

TABLE 5 Results of Evaluation Example 1 Example 2 Example 3 Example 4Center Composition (1) (1) (1) (1) Crosslinking 170 170 170 170temperature (° C.) Crosslinking time (min) 15 15 15 15 Diameter (mm) 1518 15 15 Envelope Composition (2) (2) (2) (2) layer Crosslinking 150 150150 150 temperature (° C.) Crosslinking time (min) 20 20 20 20 CoreDiameter (mm) 39.7 40.1 40.3 40.1 Volume ratio (%) 80.4 82.8 82.8 82.8Central hardness Ho 65 65 65 65 (JIS-C) Surface hardness He 86 86 86 86(JIS-C) He − Ho 21 21 21 21 Hardness distribution FIG. 2 FIG. 3 FIG. 4FIG. 4 H2-H1 (minimum value) 0.8 0.8 0.8 0.8 H2-H1 (maximum value) 4.84.8 4.0 4.0 Mid Composition (a) (a) (a) (a) layer Hardness Hm (JIS-C) 9494 94 94 Thickness (mm) 1.0 1.0 0.9 1.0 Cover Composition (b) (c) (b)(e) Hardness Hc (JIS-C) 47 56 47 45 Thickness (mm) 0.5 0.3 0.3 0.3Molecular weight of PTMG 1800 1800 1800 1500 E″ (×10⁷ Pa) 3.24 46.1 3.241.80 G″ (×10⁷ Pa) 2.77 3.23 2.77 1.02 E″/G″ 1.17 1.43 1.17 1.76 BallDeformation (mm) 2.45 2.45 2.45 2.45 W#1 Ball speed (m/s) 73.9 74.1 74.074.0 Spin (rpm) 2550 2470 2480 2500 Flight distance (m) 260.0 261.0261.0 260.5 I#5 Ball speed (m/s) 58.0 58.3 58.2 58.2 Spin (rpm) 48904800 4820 4850 Flight distance (m) 182.5 183.2 183.1 183.0 SW Dry spin(rpm) 6600 6500 6530 6550 Wet spin (rpm) 4300 4200 4280 4400 Feel atimpact A B A A

TABLE 6 Results of Evaluation Example 5 Example 6 Example 7 Example 8Center Composition (1) (1) (3) (1) Crosslinking 170 170 170 170temperature (° C.) Crosslinking time (min) 15 15 20 15 Diameter (mm) 1515 15 15 Envelope Composition (2) (2) (2) (6) layer Crosslinking 150 150150 150 temperature (° C.) Crosslinking time (min) 20 20 20 20 CoreDiameter (mm) 40.1 40.1 40.1 39.7 Volume ratio (%) 82.8 82.8 82.8 80.4Central hardness Ho 65 65 65 65 (JIS-C) Surface hardness He 86 86 86 83(JIS-C) He − Ho 21 21 21 18 Hardness distribution FIG. 4 FIG. 4 FIG. 4FIG. 5 H2-H1 (minimum value) 0.8 0.8 0.8 0.8 H2-H1 (maximum value) 4.04.0 4.0 3.0 Mid Composition (a) (a) (a) (a) layer Hardness Hm (JIS-C) 9494 94 94 Thickness (mm) 1.0 1.0 1.0 1.2 Cover Composition (f) (g) (e)(e) Hardness Hc (JIS-C) 42 42 45 45 Thickness (mm) 0.5 0.5 0.3 0.3Molecular weight of PTMG 1500 1000 1500 1500 E″ (×10⁷ Pa) 3.51 6.26 1.801.80 G″ (×10⁷ Pa) 1.42 3.00 1.02 1.02 E″/G″ 2.47 2.09 1.76 1.76 BallDeformation (mm) 2.45 2.45 2.45 2.50 W#1 Ball speed (m/s) 74.0 73.9 74.073.8 Spin (rpm) 2530 2540 2500 2460 Flight distance (m) 260.3 260.2260.5 260.0 I#5 Ball speed (m/s) 58.1 58.1 58.2 57.9 Spin (rpm) 48704890 4850 4790 Flight distance (m) 182.7 182.6 183.0 182.1 SW Dry spin(rpm) 6700 6680 6550 6490 Wet spin (rpm) 4600 4570 4400 4380 Feel atimpact A A A A

TABLE 7 Results of Evaluation Compa. Compa. Compa. Compa. Example 1Example 2 Example 3 Example 4 Center Composition (4) (1) (1) (1)Crosslinking 170 170 170 170 temperature (° C.) Crosslinking time (min)20 15 15 15 Diameter (mm) 39.7 25 15 15 Envelope Composition — (5) (6)(2) layer Crosslinking — 150 150 150 temperature (° C.) Crosslinkingtime (min) — 20 20 20 Core Diameter (mm) 39.7 39.1 38.5 40.1 Volumeratio (%) 80.4 76.8 73.3 82.8 Central hardness Ho 66 64 63 66 (JIS-C)Surface hardness He 84 87 83 86 (JIS-C) He − Ho 18 23 20 20 Hardnessdistribution FIG. 6 FIG. 7 FIG. 8 FIG. 4 H2-H1 (minimum value) 0.0 0.60.8 0.8 H2-H1 (maximum value) 2.4 5.9 3.0 4.0 Mid Composition (a) (a)(a) (a) layer Hardness Hm (JIS-C) 94 94 94 94 Thickness (mm) 1.0 1.0 1.61.0 Cover Composition (b) (b) (b) (d) Hardness Hc (JIS-C) 47 47 47 67Thickness (mm) 0.5 0.8 0.5 0.3 Molecular weight of PTMG 1800 1800 18001800 E″ (×10⁷ Pa) 3.24 3.24 3.24 7.57 G″ (×10⁷ Pa) 2.77 2.77 2.77 4.01E″/G″ 1.17 1.17 1.17 1.89 Ball Deformation (mm) 2.45 2.45 2.40 2.45 W#1Ball speed (m/s) 73.7 73.5 73.8 74.2 Spin (rpm) 2600 2650 2680 2440Flight distance (m) 259.1 258.6 258.9 259.9 I#5 Ball speed (m/s) 57.757.5 57.8 58.4 Spin (rpm) 4950 5000 4980 4810 Flight distance (m) 181.9181.3 181.7 182.9 SW Dry spin (rpm) 6630 6700 6710 6300 Wet pin (rpm)4290 4350 4370 4000 Feel at impact A A B C

As shown in Tables 5 to 7, the golf balls according to Examples areexcellent in various performance characteristics. From the results ofevaluation, advantages of the present invention are clear.

The golf ball according to the present invention can be used for playinggolf on golf courses and practicing at driving ranges. The abovedescriptions are merely for illustrative examples, and variousmodifications can be made without departing from the principles of thepresent invention.

1. A golf ball comprising a core, a mid layer positioned outside thecore, and a cover positioned outside the mid layer, wherein the corecomprises a center and an envelope layer positioned outside the center,a ratio of a volume of the core to a volume of a phantom sphere of thegolf ball is equal to or greater than 76%, a difference (He−Ho) betweena JIS-C hardness He at a surface of the core and a JIS-C hardness Ho ata central point of the core is equal to or greater than 15 but equal toor less than 30, a JIS-C hardness Hc of the cover is less than thehardness Ho, at all points P included in a zone that extends over adistance range from equal to or greater than 1 mm to equal to or lessthan 15 mm from the central point of the core, the followingmathematical formula is satisfied,−5<H2−H1<5 in the mathematical formula, H1 indicates a JIS-C hardness ata point P1 that is located radially inward of each point P at a distanceof 1 mm from the point P, and H2 indicates a JIS-C hardness at a pointP2 that is located radially outward of each point P at a distance of 1mm from the point P.
 2. The golf ball according to claim 1, wherein theJIS-C hardness Hc of the cover is equal to or less than
 65. 3. The golfball according to claim 1, wherein a thickness of the cover is equal toor less than 0.8 mm.
 4. The golf ball according to claim 1, wherein aJIS-C hardness Hm of the mid layer is equal to or greater than
 90. 5.The golf ball according to claim 1, wherein a thickness of the mid layeris equal to or less than 1.5 mm.
 6. The golf ball according to claim 1,wherein the cover is formed from a resin composition, and a principalcomponent of a base material of the resin composition is a thermoplasticpolyurethane.
 7. The golf ball according to claim 6, wherein a shearloss elastic modulus G″ of the resin composition, which is measuredunder conditions of a vibration frequency of 10 Hz and a temperature of0° C., is equal to or less than 1.95×10⁷ Pa, and a ratio (E″/G″) of atensile loss elastic modulus E″ of the resin composition, which ismeasured under conditions of a vibration frequency of 10 Hz and atemperature of 0° C., to the shear loss elastic modulus G″ is equal toor greater than 1.76.
 8. The golf ball according to claim 7, wherein thetensile loss elastic modulus E″ is equal to or greater 2.00×10⁷ Pa butequal to or less than 1.00×10⁸ Pa.
 9. The golf ball according to claim6, wherein a polyol component of the thermoplastic polyurethane ispolytetramethylene ether glycol having a number average molecular weightof 1500 or less.
 10. The golf ball according to claim 6, wherein anisocyanate component of the thermoplastic polyurethane is4,4′-dicyclohexylmethane diisocyanate.
 11. The golf ball according toclaim 1, wherein a diameter of the center is equal to or greater than 10mm but equal to or less than 20 mm, a thickness of the envelope layer isequal to or greater than 8 mm but equal to or less than 18 mm, athickness of the mid layer is equal to or less than 1.2 mm, and athickness of the cover is equal to or less than 0.5 mm.
 12. The golfball according to claim 1, wherein a difference (Ho−Hc) between thehardness Ho and the hardness Hc is equal to or greater than 15 but equalto or less than
 40. 13. The golf ball according to claim 1, wherein thecenter is formed by a rubber composition being crosslinked, and therubber composition of the center does not include an organic sulfurcompound, and the envelope layer is formed by a rubber composition beingcrosslinked, and the rubber composition of the envelope layer includesan organic sulfur compound.
 14. The golf ball according to claim 1,further comprising an adhesive layer positioned between the mid layerand the cover, wherein the adhesive layer is formed from an adhesive, abase polymer of the adhesive is a two-component curing type epoxy resinobtained by curing a bisphenol A type epoxy resin with a curing agentincluding a polyamine compound, and a gel fraction of the adhesive isequal to or greater than 40% but equal to or less than 80%.
 15. The golfball according to claim 14, wherein a ratio of an epoxy equivalent ofthe bisphenol A type epoxy resin to an amine active hydrogen equivalentof the curing agent in the adhesive is equal to or greater than 2.0/1.0but equal to or less than 13.0/1.0.
 16. The golf ball according to claim14, wherein a thickness of the adhesive layer is equal to or greaterthan 0.001 mm but equal to or less than 0.1 mm.
 17. The golf ballaccording to claim 1, wherein the hardness Ho is equal to or greaterthan 40 but equal to or less than
 80. 18. The golf ball according toclaim 1, wherein the hardness He is equal to or greater than 70 butequal to or less than
 90. 19. The golf ball according to claim 1,wherein a difference (He−Hi) between the hardness He and a JIS-Chardness Hi at an innermost portion of the envelope layer is equal to orgreater than 10 but equal to or less than
 25. 20. The golf ballaccording to claim 1, wherein at all the points P included in the zonethat extends over the distance range from equal to or greater than 1 mmto equal to or less than 15 mm from the central point of the core, thefollowing mathematical formula is satisfied,0<H2−H1<3.